The present invention relates to a method for preparing a mask pattern and a photomask prepared by the method above.
With a recent advance toward a high integration of LSIs, a transfer pattern has been more and more miniaturized. That is, a recent tendency has been toward the use of short wavelength of a light source in an exposure apparatus, the use of a high numerical aperture in an optical projection system, the use of a high resolution resist, the use of a super resolution technique, etc. By the use of such techniques, a pattern which is very fine from an optical viewpoint in comparison with a conventional pattern, that is, a pattern which is low in a k1 value, can be transferred onto a wafer, noting that the k1 value denotes a value normalized by .lambda./NA where .lambda.: a light exposure wavelength and NA: the numerical aperture. In the case where such a low-k1 pattern is transferred onto the wafer, an optical proximity effect (OPE), that is, a displacement of a transfer pattern from a desired pattern caused by a pattern arrangement, etc., becomes prominent. In recent years, investigation has been conducted on the use of an optical proximity correction (OPC) as a technique for obtaining a transfer pattern, as desired, on the wafer by considering such optical proximity effect at a designing stage and initially correcting an associated mask.
One OPC method is by finding an optical image on a wafer by virtue of a photolithography computation and calculating an amount of displacement between a desired pattern and a transfer pattern on the basis of the optical image found. With respect to the optical proximity effect resulting from an optical system and resist process, it is only necessary to perform computation on an area in a radius range of below 1 to 2 .mu.m. If, on the other hand, a finishing configuration, after etching, is made to have a desired configuration, calculation is required in a range of about a few .mu.m to several tens of .mu.m.
Design data as a correction target often takes a form such that the same pattern is repeatedly arranged. In this case, the data structure is divided into drawing information (apex coordinate information) and array information of the same pattern, so that it is possible to largely compress the data amount. In a memory device such as a DRAM, therefore, a data hierarchical structure is often deep.
FIG. 7 shows a practical model as a basic hierarchical structure where the same cells are arranged in a given array. In this practical structure, an intermediate layer cell is comprised of (2.times.2) lowest layer cells and a highest layer cell is comprised of such intermediate layer cells, that is, (4.times.4) intermediate layer cells. If, in order to set the finishing configuration, after etching, in a desired state, the optical proximity correction is done on such data while consideration is given up to the etching, a larger portion of such a layer structure of the design data is flattened.
If the optical proximity correction is made in a range of a few .mu.m to a few tens of .mu.m while consideration is given up to the etching, then a problem arises, such as it will be very difficult to prepare a mask because associated data is explosively grown.